The present invention relates to a method for obtaining high-purity carbon monoxide from a feed gas, such as off-gases from converters furnace or blast furnaces, containing carbon dioxide and nitrogen in addition to carbon monoxide by a modification of the pressure swing adsorption (PSA) technique.
Off-gases from refining vessels used in iron mills containing farily large amounts of carbon monoxide. The chemical composition of off-gases from a converter and a blast furnace are listed below.
______________________________________ CO CO.sub.2 N.sub.2 H.sub.2 ______________________________________ Off-gas from 60-87% 3-20% 3-20% 1-10% converter furnace Off-gas from 20-30% 20-30% 40-60% 1-10% blast furnace ______________________________________
High-purity CO recovered from these off-gases at low cost could be used as a raw material for synthesis of chemicals or as a gas to be blown into molten metal in refining vessels. Most reactions for the synthesis of chemicals require high temperatures and pressures, and therefore, the CO used should have the lowest possible content of C0.sub.2 that corrodes the reactor by oxidation. In order to ensure a high reaction efficiency, N.sub.2 that usually does not take part in the reaction should be removed as much as possible. While various gases are blown into a refining vessel for the purpose of increasing the efficiency of metal refining, argon that is expensive is typically used in order to avoid the increase in the concentrations of impure gases (e.g. H.sub.2 and N.sub.2) in the molten metal. Since off-gases are produced in large quantities from converters and blast furnaces in an iron mill, high-purity CO recovered from these gases at low cost could be used as an almost equally effective alternative to argon. In this case, the nitrogen content of the high-purity CO should be as low as possible for the purpose of preventing the increase in the N.sub.2 content of molten iron. Furthermore, the C0.sub.2 concentration should desirably be low in order to prevent oxidative attack of the carbon-base refractory lining of refining vessesl.
It has been proposed to recover high-purity CO from off-gases from iron mills either by deep-freezing separation or by solution absorption techniques such as the copper solution method and cosorb method. However, the deep-freezing separation technique requires low temperature and high pressure, whereas the solution absorption technique requires high temperature and pressure. Furthermore, both techniques need complicated and expensive equipment. Another disadvantage with the deep-freezing technique is that the boiling points of N.sub.2 and CO are so close to each other that their complete separation is very difficult.
Therefore, the present inventors looked to adsorption techniques for recovering high-purity CO by a simpler and less expensive process. The objective of the inventors was to recover high-purity CO not only from off-gases from refining furnaces but also from off-gases from petroleum refineries and chemical synthesis plants, as well as from off-gases resulting from the partial oxidation or hydroreforming of natural gas and heavy hydrocarbon oils.
Separation of gases by pressure-swing adsorption (PSA) is known, and methods for recovery of less strongly adsorbable gases (those components which are not easily adsorbed on an adsorbent) are shown in Japanese Patent Publication Nos. 23928/63 and 15045/68. However, as far as the inventors know, nobody has succeeded in recovery of high-purity CO from off-gases containing not only nitrogen but also carbon dioxide which has a high tendency to be adsorbed together with carbon monoxide.
U.S. Ser. No. 517,272 by Matsui et al filed on July 26, 1983, now U.S. Pat. No. 4,468,238, which was assigned to the assignee of this invention, discloses removal of nitrogen from mixture comprising nitrogen and carbon monoxide or nitrogen, carbon monoxide and carbon dioxide. However, Matsui et al does not disclose separation of carbon dioxide from said mixture.